In recent years, research and development of inorganic scintillators have been rapidly progressed, and the performance of the inorganic scintillators has dramatically improved. Amongst them, an LaBr3 scintillator is excellent not only in time resolution but also in energy resolution, and has characteristics of 3% or less with respect to γ rays of Cs-137 (662 KeV). In addition, it is possible to prepare large crystals, and a measurement with high detection efficiency may be expected by increasing a peak-to-Compton ratio. The effective atomic number of the LaBr3 scintillator is somewhat low. However, because of the high density of 5.3 g/cm3, the LaBr3 scintillator is suitable for measurement of high energy γ rays, and attracts attention as an alternative detector to a high purity Ge detector.
Meanwhile, since the LaBr3 scintillator has γ rays (1,436 keV) from 138La as self-radioactivity and residual radioactivity of 227Ac series (releasing α rays of 5 to 6 MeV), the self-radioactivity causes a large measurement error in the measurement of γ rays of low activity. In particular, since there is a spectrum associated with α decay in a region corresponding to 1.7 to 2.4 MeV, the characteristics of the LaBr3 scintillator may not be fully utilized in the high energy γ ray region.
FIG. 12 is a self-radioactivity spectrum of LaBr3:Ce. In the figure, the horizontal axis represents energy (MeV) and the vertical axis represents event frequency. It can be seen that there are many spectra associated with α decay at 1.7 to 2.4 MeV. When an event to be measured is other than 1.5 to 2.5 MeV, it is considered that the background (BG) of α decay is not a particular problem, but a large amount of environmental radiation or a large number of nuclear reactions are distributed, which may be problematic because they often have a low counting rate.
Although it may be considered that it is sufficient to simply subtract the background (BG) of α decay, it takes time to obtain sufficient statistics in the event of a low counting rate. In addition, since the detection efficiency for high energy gamma rays is generally low, the counting rate is often low.
Non Patent Document 1 below describes a method of rejecting a ray events by comparing a partial charge amount and a total charge amount using a difference in an emission signal of an LaBr3 scintillator between γ ray events and the α ray events.